The Impact of Prenatal Vitamin D Supplementation on Prenatal Vitamin D concentrations and Early Childhood Caries. Abstract Alaska Native (AN) children experience high rates of early rickets and vitamin D deficiency compared with other US populations. Pregnant AN women from Alaska`s Yukon Kuskokwim (YK) Delta have increased rates of vitamin D deficiency associated with transition from a traditional marine diet. Alaska Native children from the YK Delta experience one of the highest reported rates of early childhood caries (ECC). A Canadian study showed an association between low 25-hydroxyvitamin D (25(OH)D) levels in prenatal women and ECC in their infants. We evaluated the association between YK Delta prenatal 25(OH)D levels and ECC in their children using data from the Maternal Organics Monitoring (MOM) Study?, and children's electronic dental records to assess ECC by decayed missing and filled teeth (dmft) scores. In the analysis, children 24-35 months of age with cord blood 25(OH)D <12 ng/ml had a mean dmft score 2.4 times higher than non-deficient? children (p=0.003). There is limited information on the impact of prenatal supplementation improving vitamin D concentrations in pregnant women and no information on the impact on ECC. With available evidence, it is unclear whether vitamin D supplementation should be prescribed as part of routine antenatal care. Therefore, the current American College of Obstetricians and Gynecologists recommends only the 400 IU of vitamin D in prenatal vitamins. Data in YK Delta women from the MOM study (2000-2010) showed 91% of 25(OH)D cord blood levels were <20ng/ml (insufficient), and 53% were <12ng/ml (deficient). In Fall 2015, YK Delta Regional Hospital (YKDRH) evaluated 25(OH)D in prenatal women ? 60% were insufficient (<20ng/ml). In Fall 2016 YKDRH implemented guidelines to add 1000 IU of daily vitamin D to routinely recommended prenatal vitamins (400 IU) and calcium (200 IU). The recent Vitamin D supplementation guidelines in YK Delta offers an opportunity to evaluate the impact of prenatal supplementation on 25(OH)D concentrations and ECC. We propose a cohort study using the electronic health records and dental records to evaluate: 1) the effectiveness of prenatal vitamin D supplementation in increasing 25(OH)D concentrations to sufficient levels, 2) the association between deficient prenatal (25(OH)D concentrations and ECC, and 3) the impact of prenatal vitamin D supplementation on ECC. We will test the difference in 25(OH)D levels at first prenatal visit and third trimester in women before and after enhanced vitamin D supplementation, and measure factors associated with compliance. We will evaluate mean dmft scores between non-deficient and deficient Vitamin D groups, and before and after supplementation. and explore the relationship of covariates. This Information will help clinicians make decisions about prenatal vitamin D supplementation in YK Delta and other high risk populations.